The abundance of H2D+ in prestellar cores in Corona Australis

Coordinator: J. Harju, L. Haikala, K. Mattila

Abstract:
We propose the measurement of the H2 D+ (110 111 ) line at 372 GHz with the APEX- 2a receiver towards the centres of four prestellar cores in the R Coronae Australis cloud. H2 D+ is likely to be one of the principal molecular ions in prestellar cores and has a central role in their chemistry. Furthermore, H2 D+ may be the only observable species in the dense nuclei of low-mass cores where compounds containing heavy elements are depleted. No other transitions of H2 D+ than 372 GHz can be observed from the ground and even that one lies between two atmospheric absortion lines (O2 and H2 O) and requires the best possible observing conditions. The atmospheric transmission at this frequency at APEX is about 0.7.
The selection of cores for the proposed observations is based on previous molecular line and dust continuum studies with the SEST/SIMBA and ATCA. The physical conditions and chemical composition (including the degree of depletion) change from core to core, probably depending on the external conditions and the stage of evolution. The cloud is therefore well suited for studying the dependence of the H2 D+ abundance on physical conditions and the degree of depletion. The goals of the proposed observations are 1) to obtain quantitive information on the suggested enhancement of H2 D+ in depleted cores; and 2) to test if H2 D+ mapping with APEX can be used to localize and study chemically desolate, dense nuclei of pre-stellar cores, i.e. sites of future star formation.
Corona Australis (R.A.=19h) culminates at midnight in July. The proposed pro- gramme consists of 1) four single pointings and 2) one nine-point map in case the signal is sufficiently strong. The total time needed to complete both parts of the programme is 4 hours.

Heavy elements such as C, O and N, are supposed to be nearly completely depleted in cold, dense nuclei of low-mass pre-stellar cores, and in these conditions the chemistry is likely to be dominated by various forms of hydrogen, e.g. H2 , H+2 and H+3 and their deuterated forms (Walmsley et al. 2004). Deuterium fractionation reactions are favoured in cold gas and these result in high abundances of H2D+ , D3H+ and D3+. The effect is suggested to be accentuated in regions where CO and other heavier molecules are depleted (Caselli et al. 2003; Vastel et al. 2004).
The H2 D+ ion has a central role in transferring deuterium to other species via ion-molecule reactions. The modelling results of Walmsley et al (2004) suggest that the H 2 D+ /H+3 abundance ratio depends on the grain size distribution and the degree of ionization. For these reasons,the H2 D+ abundance and the H2 D+ /H+3 ratio are extremely useful parameters characterizing the chemical and physical state of a dense core. Furthermore, as pointed out by Bergin et al. (2002), H2 D+ may be the only available tracer of dense interiors of a pre-stellar core just before its collapse.
The goal of the proposed programme is twofold. First, we wish to determine the fractional abundance of H2 D+ in selected cores, where the physical charactistics, the degree of depletion, and the degree of deuterium fractionation for some other key molecules have been determined. With the aid of these data it would be possible to examine directly the suggested enhancement of H2 D+ in depleted regions. The second goal is to test the feasibility of 372 GHz H2 D+ line mapping as a tool for localizing and studying the nuclei of pre-stellar cores.

Dense cores in Corona Australis
The R Coronae Australis cloud (distance 170 pc) contains an active, low- to intermediate-mass star forming region in its northwestern part and a quiescent, massive pre-stellar core at its southeastern end. The large-scale structure of the cloud has been studied, e.g. by Harju et al. (1993, SEST/C18 O) and Chini et al. (2003, SEST/SIMBA). The difference in the star formation activity between different parts of the cloud is reflected in the physical conditionsand chemical composition of the dense cores. The cores have different kinetic temperatures, and different degrees of molecular depletion and deuterium fractionation (Anderson et al. 1999;Kontinen et al. 2000,2003). The cloud provides therefore an excellent opportunity to study the dependence of the H2 D+ abundance on physical conditions and examine the suggested enhancement of H2 D+ in depleted regions.
For the proposed survey with APEX we have selected four locations which correspond to the centres of dense cores characterized in the attached table.

Core

RA

DEC

Tkin

Notes

RCrA NW

19:01:47.7

-36:55:15

15-18 K

NH3 and DCO+ peak, close to MMS10

RCrA Centre

19:01:52.6

-36:57:48

21-24 K

NH3 peak, close to MMS13

RCrA SE

19:02:12.9

-37:00:36

10-13 K

DCO+ peak

CrA S

19:03:56.0

-37:15:30

10 K

N2 H+ peak, undepleted massive core, MMS25

The kinetic temperatures are derived from CH3 CCH and NH3 observations. The "MMS" sources refer to 1.2mm dust continuun peaks identified by Chini et al. (2003). In the northwestern and central parts of the cloud these peaks may represent dust temperature maxima. The positions in RCrA NW and RCrA Centre have been therefore chosen on the basis of high resolution NH 3 maps with ATCA (Harju et al., in preparation). The positions in RCrA SE and and CrAS are based on the SEST maps in N2 H+ and DCO+ (Anderson et al. 1999; Kontinen et al. 2003). N2 H+ and NH3 which can probably withstand depletion longer than most other molecules and pin-point the density peaks (Aikawa et al. 2003; Tafalla et al.2002; Hotzel et al. 2004).

Observing plan and time estimate
We propose the following observations with the APEX-2a receiver tuned to the frequency of the H2 D+ (1_10 1_11 ) line (372421.34 MHz):
1. Single pointings towards the four locations listed above.
2. A nine-point map with a 17 spacing (one HPBW) centered on the position with the strongest signal, if TA 1.
The OFF position can be chosen 2m 30s west (in R.A.) of each ON position.
The bandwidth 128 MHz gives a sufficient spectral resolution (62.5 kHz or 50 m/s) as the thermal linewidth of H2 D+ at 10 K is about 340 m/s. We estimate that an RMS noise level of 0.1 K is adequate for the first part of this survey. On the basis of the H2 column densities derived towards the objects, and the previous H2 D+ measurements of Caselli et al. (2003) towards L1544 with the CSO, we expect the peak antenna temperatures, TA , to lie in the range 0.5 - 1.0 K.
According to atmospheric transmission model of Pardo et al. (2004) the zenith opacity at 372 GHz on Chajnantor is about 0.3 under good weather conditions. As the APEX-2A receiver operates in the DSB mode, the system temperature, TSYS , is affected by the atmospheric trans- mission in both side bands. Of the two absorption bands lying on both sides of the H 2 D+ line frequency that of O2 (at about 368 GHz) is weaker, and it seems reasonable to put the line to the USB. Depending on the IF frequency the contribution of the atmospheric emission from LSB will still be considerable. Using the formula given on the APEX web pages we estimate that the equivalent system temperature outside the atmosphere (DSB) will lie in the range 400 - 550 K at elevation 60 . The observations should be performed above elevation 50 . In these conditions the ON-source integration time needed to reach the required RMS level 0.1 K is 12 minutes. Allowing for OFF measurements and telescope movements the observing time per position is 30 minutes. The total observing time needed for four single pointings is thus 2 hours.
The nine-point map should be made only if the maximum signal exceeds TA = 1 K. In this case it is sufficient to reach an RMS level of 0.2 K and set the ON-source integration time to about 4 minutes. The completion of the map should then take about 1.5 hours. We estimate that 30 minutes should be allocated for tuning, pointing and focusing, and so the total time requested for the whole programme is 4 hours.